Already he has 58.8 gigabytes of material stored on his computer, he tells me impressively.

Derek (this is not his real name -- he says he doesn't want me to use his real name "for obvious reasons" that he never gets round to explaining) considers himself a survivalist.

The survivalist movement grew up in America in the 1960s. Encouraged by Cold War-era government's calls to build nuclear fallout shelters, and concerns over currency devaluation, individuals and groups began to take steps to prepare themselves against the worst.

Many survivalists in the U.S. relocate to the northwestern state of Idaho, stockpiling food, and quite often guns and ammunition, and learning how to be self-sufficient in order to survive or "disappear."

To those who have heard of it at all, survivalism is sometimes associated with extremist views. In the U.S., the movement has occasionally been hijacked by far-right groups attracted by its rejection of much of government and its fierce defense of the right to bear arms.

For example, Oklahoma bomber Timothy McVeigh was obsessed with survivalism as a teenager, setting up a generator and a store of canned food and potable water in his basement.

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For defenders of the movement, like Jim Rawles who runs a survivalist blog and lives "in a very lightly populated region west of the Rockies" this perversion by a "lunatic fringe" distorts the true message of survivalism, which is, in many ways, just about personal freedom.

Derek, 60, who moved from London to the countryside in the southeast of England four years ago, puts it another way.

"There's going to be absolute pandemonium when it does happen, so I just want to be prepared so that I'm not a burden on anyone," he says.

What this disaster might be is anyone's guess, says Derek, but he's got his hunches.

Climate change is high up on the list. Also up there is the fallout from a global economic collapse, possibly resulting from a state of peak oil -- the point where oil production reaches its peak and thereafter goes into freefall.

Even so, Derek suspects he may not live to see the meltdown he predicts is on its way.

This is perhaps why his own preparations are rather spartan. Aside from the survival manual, he has a backpack filled with a few essentials - what survivalists term a "bug-out." He keeps the rucksack in the trunk of his car; it contains a stove, dried food, blankets, boots, clothes and "a spare set of me and the wife's pills."

Jim Rawles is taking no such chances. A former U.S. army intelligence officer, he lives on a ranch in an undisclosed location with his wife (who he refers to in his blog affectionately as "the Memsahib") and their children.

Their life is almost entirely self-sufficient: They keep livestock, hunt elk and the children are schooled at home. Stored away in the ranch somewhere is a three-year supply of food.

For a city dweller it sounds almost idyllic, though Rawles -- a gently spoken and affable man -- insists it's a lot of hard work.

"The majority of survivalists live in suburban areas and they see a life away from that as an ideal," he says. "Unfortunately, from a practical standpoint it's not possible so I think for some of these people we're living out their fantasies."

When he's not looking after the ranch or re-ordering the food supply, he devotes much of his time to the blog, which he says now receives up to 70,000 visits a week.

A life-long devotee of survivalism -- he had his first "bug-out" packed when he was just 14 -- the 48-year-old has become an unofficial spokesman for the movement.

He has penned a number of books on preparedness, including a novel called "Patriots: Surviving the Coming Collapse." Set in the near future, it imagines a period of hyperinflation and socio-economic collapse, providing guidance on how to cope.

Although a work of fiction, Rawles believes the reality is not far off.

"I've come to the conclusion that the biggest lynchpin is the power grid. If it were to go down, either through economic collapse or a terrorist atrocity, then the cities are going to become unglued."

Of course, none of this kind of talk is that new. The nature of the threat may have changed but groups of various descriptions have been predicting a breakdown of society since biblical times -- and very occasionally they've been right.

What does seem to have changed, according to Rawles, is the type of people willing to take that threat seriously.

Not only does he believe that the movement is experiencing its largest growth since the late 1970s, but he counts among the new converts an increasing number of greens and left-wingers.

"They're worried by peak oil; the climate shift; the fragility of the economy. They share a lot of the same concerns as our conservative readers," he says.

But with so many possible doomsday scenarios to choose from, isn't it difficult to know what preparations to make?

Rawles says you need to be versatile. For example, a home shelter, he says, should be able to serve as a storm shelter against hurricanes, a pantry, a secure room for storing weapons, and as a fallout bunker in the event of nuclear attack.

It all sounds vaguely terrifying -- but Rawles insists he's not being paranoid.

"I really don't consider it alarmist, and knowing what I know about the fragility of society I wouldn't sleep soundly if I hadn't taken the preparations that I have."E-mail to a friend